AU633908B2

AU633908B2 - Process for reducing the level of soluble arsenic contaminants in an aqueous solution

Info

Publication number: AU633908B2
Application number: AU55051/90A
Authority: AU
Inventors: Serena Jane Domvile
Original assignee: Nerco Con Mine Ltd
Current assignee: Miramar Con Mine Ltd
Priority date: 1989-05-24
Filing date: 1990-05-15
Publication date: 1993-02-11
Anticipated expiration: 2010-05-15
Also published as: US5093007A; CA1333106C; AU5505190A; PE5291A1; PH27074A; SE9001849L; SE9001849D0

Description

63390 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION NAME ADDRESS OF APPLICANT: Nerco Minerals Company 8-100 Par-kway Drive Vancouver Washington 98622 United States of America NAME(S) OF INVENTOR(S): Serena J. DOMVILE K~ 4 4. V. V.

A~

i ADDRESS FOR SEEVICE: DAVIES COLLISON Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: "Process for reducing the level of soluble arsenic contaminants in an aqueous solution".

The following statement is a full description of this invention, including of performing it known to me/us:the best method

I

1A TECHNICAL FIELD The present invention relates generally to an improved process for removal of contaminants from wastewater. The process of the present invention is particularly suitable for removal of soluble arsenic n-= -ot-her---inorgan-ic and-cyani-±d contaminants from wastewater generated during mining, milling and other industrial operations.

BACKGROUND ART Metallic gold is conventionally extracted from gold-containing ores by dissolution in cyanide solutions.

During the extraction process, large quantities of liquid wastes containing appreciable concentrations of cyanide, thiocyanate, and inorganic contaminants are produced. To protect the environment and to comply with governmental regulations, it is generally necessary to treat wastewater effluents for removal of toxic inorganic and cyanide constituents prior to discharge of wastewater effluents to the environment. Wastewater effluent from numerous other industrial processes likewise requires treatment for removal or reduction of inorganic constituents.

Ores containing significant amounts of arsenopyrite (generally in excess of 1% As) are not readily amenable to cyanidation. Arsenopyrite containing ores are typically 2 concentrated by flotation and roasted durinq the milling process prior to cyanidation. Roasting releases oxidized forms of arsenic and sulfur constituents from the ore, and it produces wastewater and residues having high levels of arsenic contaminants. Mill wastewater effluent containing high levels of soluble arsenic is typically discharged to tailings retention ponds. Residues containing high levels of arsenic may be processed in arsenic reclamation plants or otherwise processed for recovery of soluble contaminants.

Several methods for removal of cyanide from gold mill effluents have been proposed and implemented. The alkaline chlorination process for the destruction of cyanide involves oxidation of cyanide by the hypochlorite ion at a basic pH. Liquid chlorine or solid calcium hypochlorite typically provides the source of hypochlorite ion. The Inco SO 2 /Air process utilizes mixtures of SO 2 02 to promote oxidation of cyanide constituents in the presence of a soluble copper catalyst and under basic pH reaction conditions. Hydrogen peroxide has also been used as an oxidizing agent for removal of cyanide constituents in conjunction with a soluble copper catalyst. Biological removal of cyanide in a two stage digestion process has also been proposed. Acidification/volatilization/reneutralization processes based upon the volatility of the hydrogen cyanide produced when cyanide solutions are acidified have been developed. Cyanide removal by adsorption on ferrous sulfide has also been utilized, requiring Fe:CN ratios of at least about 3:1.

Removal of soluble arsenic from milling wastewater is also important where ores contain appreciable amounts of arsenopyrite. Conventional processes utilize ferric sulphate to provide ferric oxide and/or hydroxide particulates in the wastewater solution for precipitation of solubilized arsenic from wastewater by adsorption.

1 -3- Conventional processes for removal of soluble arsenic contaminants by adsorption on ferric particulates can be quite costly due to the chemical reagent requirements. In addition, treatment of wastewater containing elevated levels of arsenic using conventional ferric adsorption processes may result in a dramatic reduction in wastewater throughput and unacceptable arsenic removal levels and efficiencies.

SUMMARY OF THE INVENTION According to the present invention there is provided a process for reducing the level of soluble arsenic contaminants in an aqueous solution comprising: sequentially mixing an arsenic removal sludge having one or more components selected from the group consisting of: ferric oxide particulates; ferric hydroxide particulates; ferrous oxide particulates; and ferrous hydroxide particulates with the aqueous solution in each of a plurality of discrete reaction stages, the arsenic removal sludge reducing levels of the soluble arsenic contaminants in the aqueous solution by forming ferric arsenate and/or by adsorbing the soluble arsenic contaminants onto the arsenic sludge; separating the arsenic removal sludge from the aqueous solution after each of the discrete reaction stages; and withdrawing treated liquid having reduced levels of soluble arsenic contaminants from the final discrete reaction stage.

Removal of soluble arsenic contaminants from wastewater according to the present invention involves treatment of wastewater with sludge comprising ferric (Fe III) particulates under reaction conditions promoting formation of ferric arsenates and/or adsorption of soluble arsenic contaminants onto the ferric particulates. It is believed that the process of the present invention promotes both chemical reactions, whereby soluble arsenic contaminants are converted to stable ferric arsenate compounds and physical reactions, whereby soluble arsenic contaminants are complexed with ferric 921125,p:\oper\dab,55051.spe,3 particulates by an adsorption mechanism. Multiple stage treatment, wherein sludge comprising ferric particulates is mixed with wastewater sequentially in a plurality of reaction vessels for relatively short retention times, followed by separation of solids from wastewater after each treatment stage, has been found to be most effective for removal of soluble arsenic contaminants from wastewater. Sludge withdrawn from one or more of the treatment stages may be recycled for use in subsequent treatment stages, and solids produced during ferric adsorption processes may also be utilized in one or more of the treatment stages to enhance overall process efficiency.

Oxidizing agent is preferably introduced into and mixed with wastewater prior to sludge treatment in an amount sufficient to provide substantially complete conversion of soluble arsenic (As III) to the oxidized As(V) state.

Addition of oxidizing agent is generally unnecessary in subsequent sludge treatment stages provided that substantially all of the arsenic in solution has been converted to the oxidized As(V) state prior to sludge treatment. Introduction of excess oxidizing agent additionally converts iron constituents present in solution in the ferrous (Fe II) state to the desired ferric (Fe III) state.

The multiple stage sludge treatment process of the present invention may be employed as a stand-along treatment for removal of substantial quantities of soluble arsenic contaminants from solution, or it may be utilized in conjunction with other contaminant removal technologies as a pretreatment stage. Contaminant 921125,p:\oper\dab,55051.spe4

ELU

oxidation followed by multiple stage sludge treatment according to the present invention is advantageously utilized in conjunction with a ferric adsorption process to provide substantially complete removal of soluble arsenic -a Jm ml__ -C AC..

se~ib-zz-!t- tamna at substantially reduced treatment costs. Wastewater which has undergone oxidation and multiple stage sludge treatment processes nay be introduced directly to a ferric adsorption treatment process for final contaminant removal by addition of fresh chemical reagents. Solids waste from the ferric adsorption process may be recycled to the multiple stage sludge treatment process to provide a closed system providing improved contaminant removal, while requiring reduced quantities of chemical reactants.

The integrated process of the present invention provides substantially complete conversion of soluble arsenic-andother inorgaieL contaminants to stable ferric compounds and complexes, at substantially reduced Fe:As !0 ratios compared to conventional ferric adsorption processes. Experimental results indicate that substantially complete removal of soluble arsenic may be accomplished at molar Fe:As ratios approaching about 1.0:1.0 or less. Contrary to many wastewater treatment processes for removal of soluble contaminants which require elevated pH reaction conditions, oxidation and multiple stage contaminant removal treatment according to the present invention are preferably carried out under neutral or acidic pH reaction conditions.

BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and additional features of the present invention and the manner of obtaining them will become apparent, and the invention will be best understood by reference to the following more detailed description a-r read in conjunction with the accompanying drawings, in which: Fig. 1 shows a schematic flow diagram illustrating a multiple stage sludge treatment process for removal of soluble arsenic contaminants according to the present invention; Fig. 2 shows a schematic flow diagram illustrating an improved ferric adsorption treatment process for removal of soluble arsenic contaminants from aqueous solutions according to the present invention; and Fig. 3 shows a schematic flow diagram illustrating an integrated contaminant removal process according to the present invention incorporating the advantages of the processes shown in Figs. 1 and 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The process of the present invention utilizes mixing of sludge comprising ferric (Fe III) particulates with wastewater under reaction conditions which promote both chemical reactions arsenic conta.inants, to form stable insoluble compounds, and physical reactions between soluble arsenic contaminants and ferric particulates to form stable complexes.

As used herein, the term "sludge" is defined as a particulate admixture or suspension comprising ferric oxide and/or ferric hydroxide particulates and/or ferrous oxide or hydroxide particulates. Suitable sludge may be derived from a variety of sources, such as the bottom of tailings ponds, recycle from ferric (Fe III) or ferrous (Fe II) particle-based separation processes, iron oxide-based materials, or the like. When ferrous (Fe II) materials are employed, an oxidizing agent is preferably introduced to convert ferrous solids to the desired ferric state.

921 125,p:\oper\dab,5505.spe,6 Iw In its simplest embodiment, the process of the present invention comprehends mixing of coataminated wastewater with sludge comprising ferric (Fe III) particulates, followed by liquid/solids separation to partition the contaminants in the solids fraction.

Substantial contaminant removal may be achieved directly in a tailings pond simply by mixing the wastewater with sediment collected on the pond bottom. Sediment collected on the pond bottom generally comprises ferric particulazes which updergo chemical and physical reactions with the Ediicontaminants to precipitate soluble contaminants in a stable, insoluble form. Liquid/solids mixing in t.e tailings pond is followed by a settling period to promo-e liquid/solids separation. After suitable mixing and settling periods, liquid withdrawn from the pond has reduced levels of a :r+,-tcontaminants. Sediment mixing in the tailings pond is preferably employed as a pretreatment stage in combination with a more comprehensive contaminant removal procedure, since it generally does not provide complete contaminant removal.

When retention (or collection) of wastewater in a tailings pond is used in combination with a ferric particle-based contaminant removal process such as a ferric adsorption process, enhanced removal of soluble contaminants may be achieved in a tailings pond pretreatment stage when solids residues from the ferric particle-based contaminant removal process are discharged into the tailings pond. Discharge of residues comprising ferric and/or ferrous particles to the tailings pond

I

enriches the ferric particulate content of the pond sediment and provides improved physical and chemical reaction kinetics for precipitation of soluble contaminants. Discharge of residues comprising ferric particles to an area of the tailings pond in proximity to the wastewater treatment intake means is especially a.

preferred to provide liquid/solids turbulence and contact, thereby creating a contaminant removal pretreatment stage in the tailings pond in proximity to the wastewater treatment intake means. Some solids settling is preferably permitted prior to withdrawal 6f wastewater from the tailings pond for treatment, since carry-over of solids to the wastewater treatment facility is generally undesirable.

Fig. 1 schematically illustrates a preferred embodiment of the contaminant removal process of the present invention wherein wastewater is treated sequentially in multiple sludcge treatment stages. Each treatment stage provides contact between ferric (Fe III) particulates and soluble iHr\ai contaminants under reaction conditions which promote both chemical and physical reactions resulting in the precipitation of soluble c-j L, xcontaminants. Multiple sludge treatment stages providing relatively short retention times in each stage provide significantly improved contaminant precipitation and removal compared to an equivalent retention time in a single or fewer process stages.

Following each treatment stage, liquid/solids separation is effected and **iR.ja-:c contaminants are partitioned from the wastewater with the solids fraction.

Sludge is introduced to and mixed with wastewater in wastewater conduit 10 through sludge supply conduit 13.

Sludge is provided from sludge reservoir 22, and is preferably introduced in quantities of about 1% to about v/v with influent wastewater depending, in part, on the sludge density. High sludge loading of at least v/v with influent wastewater is preferred throughout the multiple stage contaminant removal process to promote removal of soluble contaminants by chemical and physical reactions. Sludge loading may, however, be limited by sludge handling or volume constraints, and benefits may be

I

conferred with lower sludge loading levels. Liquid/solids mixing means 14 is preferably provided as an in-line mixer positioned downstream from the introduction of sludge to prokide complete liquid/solids mixing prior to introduction of the liquid/solids mixture into first stage reaction vessel 20. Mixing means 14 may be provided as a mechanical mixing means, aerator, baffles or the like providing turbulence and liquid/solids mixing. Suitable mixing means may also be provided in first stage reaction vessel 20 if necessary to promote liquid/solids contact or flocculation. The liquid/solids mixture is then conveyed through liquid/solids conduit 15 to first stage reaction ;vessel Accordinq to the embodiment illustrated in Fig. 1, first stage reaction vessel 20 serves primarily as a settling tank for liquids/solids separation. The liquid/solids mixture may alternatively be conveyed to a liquid/solids separator remote from the reaction vessel, but separation is preferably achieved in the reaction vessel itself. Separation may be achieved in first stage reaction vessel 20 simply by permitting solids to collect in a zone near the bottom of the reaction vessel. A settling or flocculating agent may be introduced into the reaction vessel to facilitate separation and settling of the particulates. As shown in Fig. i, flocculating agent is introduced into first stage reaction vessel 20 from settling agent reservoir 38 through supply conduit 16.

Flocculating agent may alternatively be introduced to and mixed with the wastewater prior to conveyance of the liquid/solids mixture to the reaction vessel.

Suitable settling and flocculating agents are well known in the art. Both anionic and cationic polymer flocculating agents, as well as aluminum and other metalbased flocculating agents are suitable for use with the process of the present invention. Liquid polymer

.A

flocculating agents 1T3'. and 1T35 available from Alchem Inc. in Burlington, Ontario, Canada, are especially preferred for use in the process of the present invention.

Although liquid/solids separation after each sludge treatment stage is an important feature of the present invention, substantial separation, that is, liquid/solids separation which is about 70% to about 100% complete is sufficient, and procedures for achieving complete liquid/solids separation need not be employed.

After first stage treatment is completed and liquids/solids separation has been effected, treated liquids are conveyed through liquid conduit 21 and sludge is introduced to and mixed with wastewater for second stage treatment in the same fashion described above with reference to first stage sludge treatment. Treatment in second, third and fourth reaction vessels 30, 40 and respectively, proceeds in substantially the same fashion as described above with reference to the first stage treatment, and reaction conditions in each of the multiple treatment stages are preferably substantially the sa-i, Treated wastewater is discharged from first, second, '"ird and fourth reaction vessels 20, 30, 40 and respectively, by means of wastewater conduits 21, 31, 41 and 51. Sludge is introduced to wastewater conduits 21, 31 and 41 through sludge conduits 23, 33 and 43, respectively. Liquid/solids mixing means 24, 34 and 44 are provided as in-line mixers for mixing of liquids and solids prior to introduction into second, third and fourth stage reaction vessels 30, 40 and 50, respectively.

Suitable mixing means may also be provided in second, third and fourth stage reaction vessels if necessary to promote liquid/solids contact or flocculation. Solids withdrawal conduits 27, 37 and 47 are provided for withdrawal of solids after treatment in the second, third and fourth treatment stages. Settling agent supply II~ I I IIII i ii

L

-24- 11. An improved process according to claim 9, additionally comprising recycling solids separated from said purified _C _I conduits 26, 36 and 46 provide settling or flocculating agent to the second, third and fourth reaction stages, respectively, from settling agent reservoir 38.

Solids withdrawn from one or more of the treatment reaction vessels may be recycled through sludge recycle conduit 39 to sludge reservoir 22 for use in subsequent sludge treatment stages. Sludge recycle provides process economies and reduces the overall volume of process solids discharge. Sludrge recycle should, however, be limited to about three to about eight, and preferably about three to about five recycle applications, since recycling sludge beyond these limits results in reduced contaminant removal and process efficiency. Excess or spent sludge may be discharged from the system through sludge disposal conduit 49.

Sludge treatment is provided in multiple, sequential stages with relatively short liquid/solids mixing times utilized in each treatment stage. Short liquid/solids mixing times on the order of about 2 minutes to about 12 minutes in each sludge treatment stage are preferred, and liquid/solids mixing times of about 4 minutes to about 8 minutes are especially preferred. The process of the present invention preferably employs from about two to about six sludge treatment stages. Three or four sludge treatment stages are especially preferred to provide substantial removal of soluble contaminants, r~it~u2z-.'; from wastewater.

Introduction of an oxidizing agent prior to sludge treatment is preferred to facilitate removal of arsenic contaminants from solution. Complete mixing of the oxidizing agent with wastewater is preferably achieved prior to sludge addition, so that substantially all solubilized arsenic is in the oxidized As(V) state when sludge particulates are introduced. As shown in Fig. 1, oxidizing agent is conveyed to wastewater conduit 10 from

AAI

-0 0~ oxidizing agent reservoir 28 through oxidizing agent supply conduit 29. Mixing means 12 is provided as an inline mixer and located intermediate the introduction of oxidizing agent and sludge to facilitate substantially complete oxidation prior to sludge addition.

Suitable oxidizing agents are well known in the art and include hydrogen peroxide, chlorine, permanganate, and the like. Hydrogen peroxide is especially preferred for use as an oxidizing agent since it does not generate undesirable oxidation reaction by-products. Hydrogen peroxide is preferably introduced in a quantity sufficient to achieve molar H 2 0 2 :As ratios of about 0.2:1.0 to about 2:1, and most preferably about 1.0:1.0.

When the wastewater comprises significant 1 vels of soluble cyanide contaminants, a cyanide oxidatio catalyst is preferably introduced in addition to the ox' izing agent to promote conversion of free cyanide in solution to complexed metallo-cyanide forms which are ore easily oxidized. Oxidation and removal of sol le cyanide contaminants may then proceed simulta eously with oxidation of soluble arsenic and o er inorganic contaminants. Suitable cyanide idation catalysts are well known in the art and inc de, for example, copper sulfate, copper salts, and e like. Copper sulfate is an especially preferred cyan'de oxidation catalyst, due to its availability and r atively low cost. Cyanide oxidation catalyst i preferably introduced into the wastewater prior t or coincident with introduction of the oxidizing agent. As shown in Fig. 1, cyanide oxidation catalyst is c veyed from catalyst reservoir 18 through supply cond it 19 and introduced into wastewater conduit prior o addition of oxidizing agent. In-line mixer 11 is pro ided to facilitate mixing of catalyst with free cyande in solution to promote fnrnnat nn of m-t-Fllnya'nid e-empx When catalyzed oxidation of Gyanlde -4 /A Lb /3^ Q^ ^y

^NO

13arsenic contaminants occurs, oxidizing agent such as H 2 0 2 is introduced in a quantity sufficient to provide substantially complete oxidation of arsenic contaminants. Excess oxidizing agent may be provided, if necessary, to promote oxidation of ferrous (Fe II) materials to the desired ferric (Fe III) state.

Since oxidizing agents generally, and hydrogen peroxide in particular, are relatively expensive reagents, oxidation of soluble arsenic is preferably monitored by monitoring the oxidation-reduction potential of the wastewater treatment solution. Relative degrees of arsenic oxidation are proportional to the oxidation-reduction potential (ORP) of the wastewater, and the ORP of the solution may be conveniently monitored by means of voltage measurements, ORP meters, or the like. Hydrogen peroxide may then be introduced only as necessary to provide substantially complete oxidation of soluble arsenic contaminants, as well as ferrous sludge components. Since hydrogen peroxide is a strong oxidizing agent appropriate precautions must be taken to prevent leakage and the like, as is well known in the art.

Preferred reaction conditions enhance overall removal of arsenic contaminants and improve process efficiency. pH conditions during each sludge treatment stage are preferably maintained at about 8 or below, and most preferably from about 3 to about 7. The pH of influent wastewater is measured and pH adjustment is preferably achieved, if necessary, prior to mixing of oxidizing agent and sludge with the wastewater during first stage sludge treatment. Incoming wastewater having a pH from about 3 to about 9 generally does not require pH adjustment.

921125,p:\oper\dab,5505l.spe,13 Although multiple stage sludge treatment according to the process of the present invention has been described with reference to a preferred process design employing inline mixers, chemical reagent and sludge addition, it will be recognized that sludge, oxidizing agent and catalyst may be introduced directly to the reaction vessels, with both mixing and liquid/solids separation taking place directly in the reaction vessels.

Multiple stage sludge treatment as described above with reference to Fig. 1 may be used as a stand-alone process providing removal of about 50% to about 99% of soluble arsenic in raw wastewater. Multiple stage sludge treatment according to this embodiment provides substantial removal of soluble arsenic n=a44~ contaminants from solution, in a process which requires minimal equipment and materials costs and provides high throughput rates and short treatment times. Multiple stage sludge treatment may also be used in combination with other treatment processes such as a ferric adsorption process, to provide substantially complete removal of contaminants from wastewater.

Fig. 2 illustrates an improved ferric adsorption process according to the present invention which employs many of the principles set forth above. The improved ferric adsorption process may be utilized as a standalone contaminant removal process, or it may be used in combination with the multiple stage sludge treatment process described above with reference to Fig. 1, as illustrated in Fig. 3. The ferric adsorption process illustrated in Fig. 2 is similar to conventional ferric adsorption processes, but it employs process modifications designed to maximize contaminant removal and efficiency while reducing chemical reagent and equipment costs.

Oxidation of soluble arsenic -wd a i e contaminants is preferably carried out as necessary in the same fashion as described above with reference to multiple stage sludge treatment. According to the embodiment illustrated in Fig. 2, wastewater is conveyed from a raw wastewater source through wastewater conduit 10 and delivered to oxidation reaction vessel 56. Oxidizing agent is supplied to oxidation reaction vessel 56 from oxidizing agent supply reservoir 52 by means of oxidizing agent supply conduit 53. n4-oQ-x4ati:-aa-b-rypdo-ede-t-c-aT^e'-t h^'ods e cyanide contaminants in the wastewater, and alyst is preferably introduced into new wasrt er conduit 10 from catalyst supply reservoir 5 rough supply conduit prior to reaction e oxidation reaction vessel.

Alternati cyanide oxidation catalyst may be _int-rOduced directly into oxidation reaction vessel 56.

Wastewater is retained in oxidation reaction vessel 56 until substantially all soluble arsenic .nd--4.e contaminants haVe been oxidized. Oxidation of other inorganic contaminants to oxidized forms which are more susceptible to chemical and/or physical reaction may also take place. Mixing of the oxidation reaction vessel liquid contents is preferably provided to promote substantially complete oxidation. It is an important feature of the improved contaminant precipitation/removal process of the present invention that substantially complete oxidation of soluble arsenic aR4--eyta-4econtaminants is achieved prior to the contaminant removal phase of the process. Oxidation is preferably monitored by periodically measuring the oxidation-reduction potential of the liquids, as described above. pH conditions in oxidation reaction vessel 56 are preferably maintained below about 8, from about 3 to about 7, and most preferably from about 5 to about 7.

When substantially complete oxidation has been achieved, wastewater is conveyed through wastewater

A

conduit 57 to contaminant removal reaction vessel Contaminant removal reagent is introduced into the contaminant removal reaction vessel from supply reservoir 58 through supply conduit 59. Ferric sulphate (Fe 2

(SO

4 3 x H 2 0) is a preferred reagent for formation of ferric hydroxide and/or ferric oxide particulates in the reaction vessel, and is well known for its ability to remove toxic metals from solution. Ferric sulphate is preferred due to its availability and relatively low cost, but other ferric reagents are known in the art and may be used to provide ferric hydroxide and/or ferric oxide particulates for the contaminant precipitation/removal reaction. Although ferric sulfate is conventionally used i; ferric adsorption processes wherein soluble metallic contaminants are complexed with the ferric particulates and thereby removed from solution, it is believed that the ferric adsorption process of the present invention promotes both chemical and physical reactions facilitating removal of soluble contaminants from solution.

Significantly reduced quantities of ferric sulphate are required for contaminant removal by chemical and physical reactions according to the process of the present invention, compared to conventional ferric adsorption processes. Ferric sulphate is introduced into contaminant removal reaction vessel 60 in quantities sufficient to attain molar Fe:As ratios of about 0.2:1 to about 2:1, and molar ratios of about 1.0:1.0 or less typically provide substantially complete precipitation of soluble arsenic contaminants to stable ferric arsenate compounds and/or ferric hydroxide complexes. Mixing in contaminant removal reaction vessel 60 is provided as necessary to promote contact between particulates and soluble contaminants. pH conditions in reaction vessel 60 are preferably maintained below about 8, from about 3 to about 7, and most preferably from about 3 to about 5. Retention times in

IL--

(1_ contaminant removal reaction vessel 60 vary depending upon contaminant levels in the influent wastewater, but can be expected to range from about 1 to about 10 minutes, and are preferably from about 4 to about 7 minutes.

When removal of inorganic contaminants from solution by chemical and/or physical reaction is substantially complete, the liquid/solids admixture is conveyed through liquid/solids conduit 61 to neutralization reaction vessel 64. Neutralization of the liquid/solids admixture may be required prior to discharge of the treated liquid effluent to comply with treated water discharge requirements. Neutralizing agent is supplied to reaction vessel 64 as necessary from neutralizing agent supply reservoir 62 through neutralizing agent supply conduit 63. A suitable neutralizing agent, such as lime, sodium hydroxide or the like is introduced into neutralization reaction vessel 64 as necessary to elevate the pH of the liquid/solids admixture to about 6.5 to about 7.5. Mixing of neutralizing agent with the liquid/solids contents of the neutralization reaction vessel is preferably provided to accelerate and provide accurate pH adjustment.

Neutralization is unnecessary when the liquid/solids admixture withdrawn from contaminant removal reaction vessel 60 is at a substantially neutral pH.

After the neutralization reaction is complete, the liquid solids admixture is withdrawn from reaction vessel 64 and conveyed through liquid/solids conduit 65 to reaction vessel 68, wherein a suitable settling or flocculating agent is introduced to promote settling and separation of solids, Settling or flocculating agent is conveyed through supply conduit 67 from flocculating agent supply reservoir 66. A liquid polymer flocculating agent of the type described above in connection with the multiple stage sludge treatment process is preferred since it provides convenient handling characteristics. Other types of flocculating agents which are well known in the art may also be used. Mixing of liquids and solids is preferably provided to promote flocculation and facilitate liquid/solids separation.

The liquid/solids admixture is thereafter withdrawn through liquid/solids conduit 69 and introduced into liquid/solids separation means 70. Liquid/solids separation means 70 preferably comprises a clarifier or thickener such as a lamella clarifier, wherein settling of solids occurs on a plurality of angled trays or plates.

Other suitable types of clarifiers are known in the art and may be utilized with the process of the present invention. Separation means 70 preferably provides substantially complete separation of liquids and solids, and water substantially free of solids, inorganic C: V-econtaminants such as arsenic -d--yadee amn:ans-s discharged through purified liquid conduit 72. Solids are collected and withdrawn through solids discharge conduit 71. Solids withdrawn from this contaminant removal process through discharge conduit 71 contain substantial quantities of ferric particulates, and may be collected for use in the multiple stage sludge treatment process described above with reference to Fig. i.

Fig. 3 illustrates an integrated contaminant removal process according to the present invention wherein the multiple stage sludge treatment process illustrated in Fig. 1 is employed in combination with the contaminant removal process illustrated in Fig. 2. Reference numerals used in Fig. 3 refer to the elements having identical reference numerals described above with reference to Figs.

1 and 2. As shown in Fig. 3, multiple stage sludge treatment proceeds in substantially the same fashion as described above with reference to Fig. 1. Mixing means have not been illustrated for purposes of clarity, but -19should be provided as previously described. Addition of oxidizing agent, catalyst and sludge in the multiple stage sludge treatment process also proceeds as previously described. Raw wastewater in conduit 10 is treated with oxidizing agent from reservoir 28 to promote oxidation of arsenic contaminants. Sludge is introduced to wastewater through sludge supply conduits 13, 23, 33 and 43 prior to treatment in first, second, third and fourth stage reaction vessels 20, 30, 40 and 50, respectively. Reaction conditions and liquid/solids mixing times in each of the sludge treatment stages are substantially the same and are as described above with reference to Fig.l Flocculating agent is preferably provided in each sludge treatment stage from reservoir 38 to promote solids settling and separation. Solids withdrawn from each treatment stage are preferably recycled through conduit 39 to sludge reservoir 22.

Treated wastewater discharged from the multiple stage contaminant removal process through discharge conduit 51 is conveyed to contaminant removal reaction vessel 60. It is generally unnecessary to introduce additional oxidizing agent or cyanide oxidation catalyst to the wastewater prior to treatment in contaminant removal reaction vessel 60, since complete oxidation of inorganic and cyanide contaminants prior to multiple stage sludge treatment should be sufficient.

contaminant removal reagent, preferably ferric sulfate, is introduced into reaction vessel 60 from supply reservoir 58 through supply conduit 59 based upon levels of residual arsenic in solution to achieve molar Fe:As ratios of about 0.2:1 to about 2:1, and most preferably about 1.0:1.0.

Reaction 921125,p: \oper\dab,55051.spe, 19

IL

conditions and liquid/solids mixing times are substantially as described above with reference to Fig. 2.

Liquid/solids materials are thereafter conveyed to neutralization reaction vessel 64, flocculation reaction vessel 68 and liquid/solids separation medns for treatment as described above with reference to Fig. 2. Flocculating agent is preferably supplied from central reservoir 38 through supply conduit 42. Solids discharged from liquid/solids separation means 70 through solids discharge conduit 71 are preferably conveyed through sludge recycle conduit 39 to sludge reservoir 22 for utilization in the multiple stage contaminant removal process.

The integrated contaminant removal process illustrated in Fig. 3 provides substantially complete removal of soluble arsenic ad--z-a contaminants from wastewater and demonstrates significantly improved overall process efficiencies. Processing of wastewater by oxidation and multiple stage sludge treatment achieves substantial removal of soluble arsenic and cyanide contaminants at low chemical reagent consumption rates, and substantially reduces the ferric sulfate requirement for treatment in contaminant removal reaction vessel Recycling of ferric particles also provides improved process efficiency. Fluctuations in contaminant removal efficiencies in the two phases of the integrated treatment process, the multiple stage sludge treatment phase and the ferric adsorption phase, are substantially self-regulating since lower efficiencies in the multiple stage sludge treatment result in higher contaminant levels and higher ferric sulfate consumption requirements in reaction vessel which provides recycle of larger quantities of "fresh" ferric particles for multiple stage sludge treatment.

Likewise, high contaminant removal efficiencies in the multiple stage sludge treatment phase result in lower contaminant levels and lower ferric sulfate consumption requirements in reaction vessel 60, which results in recycle of smaller quantities of "fresh" ferric particles for multiple stage sludge treatment.

Although the improved process of the present invention has been described with reference to schematic flow diagrams, it will be recognized that certain mechanical devices which have not been illustrated, such as pumps, liquid and/or solids flow meters and controllers, pH meters, ORP meters and the like may be provided as necessary to monitor contaminant removal and achieve process liquid and liquid/solids flow requirements. In addition, automated control mechanisms may be incorporated so that the process proceeds in an automated fashion, requiring minimal monitocing and supervision.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein may be varied considerably without departing from the basic principles of the invention.

Claims

2. An improved process according to claim 1, wherein mixing times of about 2 minutes to about 12 minutes are provided in each of said discrete reaction stages.
3. An improved process according to claim i, wherein the process additionally comprises converting the soluble arsenic contaminants in the aqueous solution to a higher oxidation state prior to mixing the arsenic removal sludge with the aqueous solution. 30 4. An improved process according to claim 3, wherein 4 -f converting the soluble arsenic contaminants to a higher oxidation state is achieved by introducing hydrogen peroxide in the aqueous solution in an amount sufficient to achieve molar H 2 0 2 :Arsenic ratios of about 0.2:1.0 to about 2:1. 1 rAL 921125,p:\oper\dab,55051.spe,22 L I '-L -23 An improved process according to claim 3, wherein the aqueous solution additionally includes cyanide contaminants comprising free cyanide and complexed metallo-cyanides, and the process additionally comprises mixing a cyanide oxidation catalyst with the aqueous solution prior to mixing with the arsenic removal sludge to promote conversion of free cyanide to complexed metallo-cyanides.
6. An improved process according to claim 5, wherein the cyanide oxidation catalyst comprises copper sulfate.
7. An improved process according to claim i, additionally comprising recycling the arsenic removal sludge separated from the aqueous solution after each discrete reaction stage for use in subsequent reaction stages.
8. An improved process according to claim i, wherein the pH level of the aqueous solution is maintained at about 8 or less during treatment in each of the plurality of discrete reaction stages.
9. An improved process according to claim i, additionally comprising treating the treated liquid in a second phase treatment process comprising: mixing the treated liquid with ferric hydroxide and/or ferric oxide particulates derived from a contaminant removal reagent under reaction conditions which promote chemical and/or physical reaction of the inorganic contaminants with said ferric hydroxide and/or ferric oxide particulates, and subsequently separating a purified liquid which is substantially free from solids and soluble inorganic contaminants. An improved process according to claim 9, wherein the contaminant removal reagent is ferric sulfate, and said ferric sulfate is introduced in amounts sufficient to provide molar Fe:As ratios of about 1.0:1.0 or less. 921125,p:\oper dab,55051.spe,23 nh I -24-
11. An improved process according to claim 9, additionally comprising recycling solids separated from said purified liquid in said second phase treatment process for use as said sludge in subsequent discrete reaction stages.
12. An improved process for removal of inorganic and/or cyanide contaminants from an aqueous solution substantially as described and shown in the drawings. Dated this 25th day of November, 1992 NERCO CON MINE, LTD. by DAVIES COLLISON CAVE Patent Attorneys for the applicant(s) 921 125,p:\oper\dab,55051.spe.24 Is